Impurities such as sulfur, oxygen, phosphorous, and carbon are known to effect the processing, as well as the mechanical and physical properties of steel, nickel, and cobalt alloys.
For example, sulfur causes hot shortness; oxygen causes edge cracking; while sulfur and oxygen cause surface imperfections; all of which effect the yield and costs of steel manufacture.
In addition, sulfur and oxygen are known to reduce ductility and toughness; sulfur, oxygen and phosphorous are known to lower the ductile-brittle transition temperatures, and, forming characteristics; while sulfur, oxygen, phosphorous, and carbon are known to detract from soft magnetic characteristics of steels and alloys.
Because of the increasing awareness for the need to control and eliminate these impurities in steel, a great deal of activity has been focused on developing systems or techniques that fill this need. Calcium and magnesium are both excellent deoxidizers and desulfurizers, and as such some of the work has focused on these elements and has resulted in patented calcium, and magnesium alloy additives; pneumatic injection systems, and, submersion techniques for adding calcium, magnesium and their alloys.
This invention relates to a submersion technique for adding solid shapes of lighter-than-steel, volatile, low soluble, purifying agents such as calcium, and magnesium into molten steel.
Both magnesium and calcium are lighter-than-steel, have limited solubility, and boil at temperatures lower than molten steel. A possible way to add lighter-than-steel material is via some sort of submersion technique. However, because these elements boil at low temperatures and have limited solubility, rapid vaporization occurs as they are added to liquid steel, and that which is not immediately consumed by the steel rushes to the surface of the steel causing metal eruptions and/or violent explosions, flare, and, smoke. Thus, something other than simple submersion techniques are required to avoid these problems.
Prior art teaches a variety of submersion techniques which include pneumatic injection systems for adding calcium and magnesium in fine grain particulate form U.S. Pat. No. 3,998,625, and U.S. Pat. No. 4,123,258 as well as the use of containers U.S. Pat. No. 2,915,386 for protecting such additions from contact with the molten ferrous metal until some time after the container with the treating agent is submerged, or covered, by the molten metal. In one case, a cylinder containing the treating agent is submerged U.S. Pat. No. 2,595,282 into the ladle after it is filled with the molten metal, while in other cases the container filled with the treating agent is placed and attached to, U.S. Pat. No. 3,934,862, or near, U.S. Pat. No. 3,942,775 the ladle bottom prior to the filling of the ladle.
In these cases, once the container melts, or disintegrates, the addition agent is exposed to the liquid steel, and, if the addition agent is lighter-than-steel, it quickly rises to the slag, and if the addition agent is calcium or magnesium flaring, metal eruption, and smoke result with a large portion of the calcium or magnesium being wasted to the slag and the atmosphere.
Experience teaches us that `solid` shapes of pure magnesium or calcium can be submerged into high nickel alloys without these problems occuring, and small amounts of calcium can be submerged into iron with little difficulty. However, submersion of solid shapes of these elements into steel results in violent explosions when magnesium is added, and, severe metal eruptions and flaring when calcium is added. The difference noted in the activity of these elements in different steels and alloys is considered to be related to solubility differences, with their solubility in steel being the least.
For example, in the case of calcium, Sponseller, D. L., Trans Met Society AIME, Vol. 230, June 1964, shows its solubility to be very low in steel but increases substantially as the nickel or carbon content is increased.
Immediately upon calcium dissolving, it reacts with the impurities in the steel, which results in compounds that float to the surface of the steel thus removing the impurities from the molten steel.
Various methods have been used in molten iron to reduce this violent activity by slowly introducing magnesium metal into the iron under rigidly controlled systems. One of these methods for reducing the violence is to impregnate porous bodies with magnesium metal and to introduce these magnesium impregnated porous bodies into the molten ferrous metal. Under these conditions, the impregnated magnesium metal is released at a slow enough rate that the violence is held to a minimum.
Among the known porous bodies which have been used with some success for this purpose are porous coke, U.S. Pat. No. 3,321,304, carbon, graphite and ceramic bodies U.S. Pat. No. 4,083,716, such as quicklime, lump limestone or dolomite and the like.
In addition, magnesium has been infiltrated into porous iron bodies U.S. Pat. No. 3,902,892. Among these iron bodies is sponge iron in which the iron particles are very small and are sintered together to form a porous structure.
Another method mentioned as prior art in U.S. Pat. No. 3,902,892 is iron briquettes containing magnesium produced by dry pressing together iron particles and magnesium particles both of which preferably are from 4-60 mesh.
These methods, basically designed for magnesium inoculation, or, desulfurization of iron, are comparable to calcium treatment of iron. However, in all cases they are not suitable for use in steel for a number of reasons that include: the possibility of carbon pick-up from the coke; or the pick-up of exogenous inclusions from the ceramic bodies, or, hydrogen pick-up from the binders used in the castable ceramic bodies, etc., steel being more sensitive to these impurities than iron.
In addition there are limitations as to the chemical makeup of those products made with porous bodies since the amount of magnesium (or calcium) that these bodies can hold, depends upon the amount of porosity available in the bodies, or, in case of the ceramic bodies is limited to the amount of magnesium that the ceramic mix can hold and still be a castable ceramic. The specific chemical makeup is also limited to single elements, or, alloys, since the porous bodies must be submerged into a liquid bath of the element, or alloy, in order to fill the pores of the body. Mixtures of immiscible elements, therefore, cannot be used to fill the pores.
In addition, in the case of castable ceramics, elements that react with moisture, or with the binder, such as calcium, cannot be used because they would react with the moisture or the binder to destroy the strength of the casted shape, while the calcium would be partially or wholly consumed by the reaction.
In the case of pressed together briquettes of iron and magnesium (calcium), when these are added to liquid steel via normal tap stream addition methods, large quantities of smoke and flare develop with most of the magnesium (calcium) reacting with the slag or the atmosphere. When such material is packed into a sealed steel cylinder and the cylinder containing the briquettes is submerged into a filled ladle of steel, the cylinder melts exposing the briquettes all at one time. The briquettes, being lighter than the steel, quickly float to the surface causing flaring and smoke as they reach the surface of the steel, resulting in most of the magnesium (calcium) being wasted to the slag and atmosphere.
This invention, which is the submersion of, and the holding near the bottom of the liquid bath until dissolved, a soluble pressed together solid material of a particular percentage of active scavenging material that has been adjusted to, and balanced with, the dimension of said solid material in order to provide, and replace as it is being consumed, the maximum amount of active material that the steel will take, is not restricted by the aforementioned limitations, and thus the invention provides a more flexible, efficient smoke eliminating method for adding active, volatile, lighter-than-steel, additives that reduces and prevents air pollution.
To be more specific this invention is characterized by: the submersion of a solid material containing an active treating agent, not a treating agent in fine grain particulate form, into molten steel; being a mixture, and not an alloy, of at least two pressed together materials and as such can be made up of any conceivable composition, with the composition being easily controlled so as to prevent the addition of undesirable elements into the steel; shapes much larger than briquettes such as to provide the means of properly holding the additive submerged while it is being dissolved in the steel at a reduced melting rate necessary for optimum release of the calcium into the steel.